Abstract:

An over-center mechanism includes a pivoting member, rotatable about a
pivot point between first and second positions, a drive gear, having a
first radius, fixedly attached to the pivoting member at the pivot point,
a pinion gear, engaged with the drive gear, having a second radius, and
an over-center tension spring, eccentrically attached between the drive
gear and the pinion gear, to bias the pivoting member toward the first
and second positions.

Claims:

1. An over-center mechanism, comprising:a pivoting member, rotatable about
a pivot point between first and second positions;a drive gear, having a
first radius, fixedly attached to the pivoting member at the pivot
point;a pinion gear, engaged with the drive gear, having a second radius;
andan over-center tension spring, eccentrically attached between the
drive gear and the pinion gear, positioned to bias the pivoting member
toward the first and second positions.

2. An over-center mechanism in accordance with claim 1, wherein the
pivoting member is a CD tray of a printer device.

3. An over-center mechanism in accordance with claim 1, wherein the second
radius of the pinion gear is smaller than the first radius of the drive
gear.

4. An over-center mechanism in accordance with claim 3, wherein the second
radius of the pinion gear is approximately half the first radius of the
drive gear.

5. An over-center mechanism in accordance with claim 1, wherein the spring
is eccentrically attached to the drive gear and the pinion gear at points
spaced a lever arm distance from the respective pivot points thereof, and
generally opposite a gear engagement region thereof.

6. An over-center mechanism in accordance with claim 5, wherein the second
radius of the pinion gear is smaller than the first radius of the drive
gear, and the lever arm distance of the pinion gear is substantially
larger than the lever arm distance of the drive gear.

7. An over-center mechanism in accordance with claim 5, wherein the lever
arm distance of the pinion gear is substantially larger than the lever
arm distance of the drive gear.

8. An over-center mechanism in accordance with claim 1, wherein the spring
is positioned and selected to provide a biasing force sufficient to
rotate the pivoting member toward the first position or the second
position from a position that is more than about 10 degrees away from a
center position thereof.

9. An over-center mechanism in accordance with claim 1, further comprising
a stop, positioned to physically contact the pivoting member to limit its
range of motion.

10. An over-center mechanism in accordance with claim 1, further
comprising a stop, extending from at least one of the pinion gear and the
drive gear, positioned to limit the range of motion of the gears.

11. An over-center mechanism, comprising:a pivoting member configured to
rotate between first and second positions;a drive gear attached to pivot
with the pivoting member;a pinion gear engaged with the drive gear;
andcombined means for biasing the pivoting member toward the first and
second positions, and for rotationally biasing the pinion gear against
the drive gear to provide an additive moment to bias the pivoting member
toward the first and second positions.

12. An over-center mechanism in accordance with claim 11, further
comprising means for limiting the angular range of motion of the pivoting
member.

13. An over-center mechanism in accordance with claim 12, wherein the
means for limiting the angular range of motion of the pivoting member
comprises a stop, positioned to physically contact at least one of the
pivoting member and at least one of the pinion gear and the drive gear.

14. An over-center mechanism in accordance with claim 11, wherein the
combined means for biasing the pivoting member toward the first and
second positions, and rotationally biasing the pinion gear against the
drive gear comprises a tension spring, eccentrically attached between the
drive gear and the pinion gear.

15. An over-center mechanism in accordance with claim 14, wherein the
tension spring is eccentrically attached to the drive gear and the pinion
gear at points spaced a lever arm distance from the respective pivot
points thereof, and generally opposite a gear engagement region thereof.

16. An imaging device, comprising:means for applying indicia to print
media;a pivoting member, configured for holding print media, rotatable
about a pivot point between a raised position and a lowered position;a
drive gear, having a first radius, fixedly attached to the pivoting
member at the pivot point;a pinion gear, engaged with the drive gear,
having a second radius; andan over-center tension spring, eccentrically
attached between the drive gear and the pinion gear, positioned to bias
the pivoting member toward the raised and lowered positions.

18. An imaging device in accordance with claim 16, further comprising a CD
tray, removably attachable to the pivoting member when in the lowered
position, configured for positioning a CD for receiving indicia from the
means for applying indicia.

19. An imaging device in accordance with claim 16, wherein the second
radius of the pinion gear is smaller than the first radius of the drive
gear, and the spring is eccentrically attached to the drive gear and the
pinion gear at points spaced a lever arm distance from the respective
pivot points thereof, and generally opposite a gear engagement region
thereof.

20. An imaging device in accordance with claim 16, wherein the spring is
positioned and selected to provide a biasing force sufficient to rotate
the pivoting member toward the raised position or the lowered position
from a position that is more than about 10 degrees away from a center
position thereof.

Description:

BACKGROUND

[0001]The present disclosure relates generally to over-center spring
mechanisms. Over-center spring mechanisms are typically used to
mechanically hold a pivoting structure in selected resting positions
relative to a pivot point. These mechanisms typically include a tension
spring that is attached at one end to a fixed structure, and at the other
end is attached to the pivoting structure. The position of the spring is
such that the spring extends over the pivot point of the pivoting
structure at some point during the range of the motion of the pivoting
structure. Consequently, the greatest spring force is experienced at a
point where the spring applies no moment to the pivoting structure, thus
biasing the structure away from the midpoint of its motion.

[0002]The force and motion characteristics of over-center spring
mechanisms relate to the strength and size of the spring, the position of
its attachment to the pivoting structure, and other intervening structure
that may be involved in the mechanism. Where an over-center spring
mechanism is configured for vertical motion, the spring can be configured
to support the weight of the pivoting structure when in a raised
position. This can involve the use of a relatively large and strong
spring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]Various features and advantages of the present disclosure will be
apparent from the detailed description which follows, taken in
conjunction with the accompanying drawings, which together illustrate, by
way of example, features of the present disclosure, and wherein:

[0004]FIG. 1 is a side view of one embodiment of a gear-enhanced
over-center spring mechanism in accordance with the present disclosure,
showing the pivoting member in the raised position;

[0005]FIG. 2 is a side view of the gear-enhanced over-center spring
mechanism of FIG. 1, showing the pivoting member in a partially lowered,
but not yet past center position;

[0006]FIG. 3 is a side view of the gear-enhanced over-center spring
mechanism of FIG. 1, showing the pivoting member in a partially lowered
and slightly past center position;

[0007]FIG. 4 is a side view of the gear-enhanced over-center spring
mechanism of FIG. 1, showing the pivoting member in the lowered position;

[0008]FIG. 5 is a free body diagram of one embodiment of the pivoting
member;

[0009]FIG. 6 is a free body diagram of one embodiment of the drive gear;

[0010]FIG. 7 is a free body diagram of one embodiment of the pinion gear;

[0011]FIG. 8 is a perspective view of a printer device having a moveable
CD tray that can be actuated using one embodiment of a gear-enhanced
over-center spring mechanism in accordance with the present disclosure,
showing the CD tray in the raised position; and

[0012]FIG. 9 is a perspective view of the printer device of FIG. 8, with
the CD tray in the lowered position and a CD carrier installed.

DETAILED DESCRIPTION

[0013]Reference will now be made to exemplary embodiments illustrated in
the drawings, and specific language will be used herein to describe the
same. It will nevertheless be understood that no limitation of the scope
of the present disclosure is thereby intended. Alterations and further
modifications of the features illustrated herein, and additional
applications of the principles illustrated herein, which would occur to
one skilled in the relevant art and having possession of this disclosure,
are to be considered within the scope of this disclosure.

[0014]As used herein, the terms "lever arm" and "moment arm" are distinct.
The term "lever arm" is intended to refer to the physical distance from a
pivot point of a member to a point of action of an eccentric or rotating
force upon that member. The term "moment arm" is intended to refer to the
distance between the line of action of an eccentric force upon a pivoting
member and a line passing through the pivot point, the distance being
measured perpendicularly to the line of the force vector.

[0015]The term "over center spring" is recognized by those skilled in the
art as referring to a spring that is attached to a pivoting structure,
and positioned to rotate or translate across a pivot point or center
point as the pivoting structure rotates. An over center spring provides a
biasing force upon the pivoting structure in one direction while on a
first side of the center point, the biasing force dropping to zero as the
spring rotates or translates to the center point, after which the spring
provides a biasing force on the pivoting member in the opposite direction
on a second side of the center point.

[0016]As noted above, over-center spring mechanisms are typically used to
mechanically hold a pivoting device such as a door or the like in
selected resting positions relative to a pivot point. There are cases
where the characteristics of an over-center spring mechanism can be more
desirable than other devices such as buckling blades or spring loaded cam
blocks. However, there are certain design considerations that affect
over-center spring mechanisms. For example, where the pivoting device
pivots upwardly, it can be desirable for the force provided by the
over-center spring mechanism to be sufficient to support the weight of
the pivoting structure when in the raised position. The angular
separation of the desired resting positions of the pivoting structure can
also be significant in some cases, and this angular separation has an
influence on the holding force provided by the spring. Additionally, the
appearance and space to contain the over-center spring mechanism can be
significant in some cases.

[0017]Advantageously, a gear-enhanced over-center spring mechanism has
been developed that provides smooth action and reduces the relative
amount of spring force that is needed to properly bias the pivoting
structure in the desired positions. While this device can be used in many
applications, one application for this mechanism that has been used is
for a flip-down CD tray for an imaging device such as an ink jet printer.

[0018]One embodiment of a gear-enhanced over-center spring mechanism in
accordance with this disclosure is depicted in FIGS. 1-4. The over-center
spring mechanism generally includes a pivoting member 10, which is
pivotally attached at fixed pivot point 12 and can rotate between a first
position, indicated at 14, and a second position, shown in dashed lines
at 16. An upper stop 18 and lower stop 20 can be attached to an adjacent
fixed structure and positioned to contact the pivoting member to
physically limit its range of travel.

[0019]The pivot point 12 of the pivoting member 10 is supported by a
support structure or frame 22. Attached to the pivoting member at the
pivot point is a drive gear 24 with gear teeth 26 located along a portion
of the outside edge of the gear. The drive gear is fixedly attached to
the pivoting axle of the pivoting member, and thus rotates in concert
with the pivoting member. A pinion gear 28 is also attached to the
support structure at a pivot point 30, and includes gear teeth 32 that
are located along a portion of the outside edge of the gear and engaged
with the teeth of the drive gear. As an alternative to the upper and
lower stops 18 and 20, the pinion gear can include a left stop 19 and
right stop 21, shown in dashed lines in FIG. 3. These stops on the gear
surface block the gears from continued rotation once a certain angular
position is reached. While two stops are shown on the pinion gear, it is
to be understood that stops can alternatively be associated with the
drive gear, and either of the gears can be provided with just one stop if
desired. Any structure that physically limits the range of angular travel
of the pivoting member can function as a stop, whether such structure is
associated with the gears, the pivoting member, or some other adjacent
structure.

[0020]The drive gear 24 includes a spring attachment tab 34 that is
located generally on an opposite side of the drive gear pivot point 12
from the gear teeth 26 of the drive gear. Likewise, the pinion gear 28
includes a tab 36 that is located generally on an opposite side of the
pinion gear pivot point 30 from the gear teeth 32 of the pinion gear. An
over-center tension spring 38 is attached between the tabs of the drive
gear and the pinion gear, and applies a force that tends to rotate the
two gears in opposite directions.

[0021]The drive gear 24 can have a pitch or gear radius that is
substantially larger than the pitch radius of the pinion gear 28. In one
embodiment, the pinion gear has a pitch radius that is approximately half
the pitch radius of the drive gear. Consequently, when the drive gear
rotates a given angular distance, the pinion gear will be caused to
rotate a substantially larger angular distance. The distance of the
spring attachment tabs 34 and 36 from the pivot points of the respective
gears provide an eccentric attachment point for the spring, which creates
a moment arm for the force of the tension spring 38, which causes the
tension spring to apply a torque or moment to the gears. As the gears
rotate, the angle of the spring will change and the respective moment arm
distances will vary, thus causing the torque applied by the spring to
vary. For reasons that will become more clear hereafter, the spring lever
arm length of the pinion gear 28 can be substantially larger than the
spring lever arm length of the drive gear 24. It is to be appreciated
that while the embodiment shown in the figures includes a drive gear with
a radius that is substantially larger than the radius of the pinion gear,
this is only one of many possible embodiments. The relative radii of the
drive gear and pinion gear can vary while still providing a functional
gear-enhanced over-center mechanism. For example, the drive gear and
pinion gear can have the same radius, or the pinion gear can be larger
than the drive gear.

[0022]As the pivoting member 10 is rotated, the drive gear 24, pinion gear
28 and over-center spring 38 act to bias the pivoting member 10 toward
the first position 14 and the second position 16. The operation of the
mechanism is illustrated in FIGS. 1-4. In the condition shown in FIG. 1,
the pivoting member is in the fully up position 14. In this position, the
pinion gear and drive gear are rotated so that the spring attachment tabs
34, 36 place the over-center spring 38 to the left of the center points
of the gears. In this position the moment arms of the drive gear and
pinion gear are at their maximum length, which tends to increase the
torque provided by the spring.

[0023]Because the drive gear 24 is fixedly attached to the axle of the
pivoting member 10 and the spring 38 is attached to the tab 34 on the
drive gear, the drive gear acts as a lever so that the spring exerts a
lifting force on the pivoting member to hold the pivoting member in the
up position. At the same time, the spring exerts a counter-rotating force
on the pinion gear 28, which, through the gear mesh, provides a
rotational force on the drive gear to provide additional lift to the
pivoting member. It has been found that the gear train of this
over-center spring mechanism provides enough additional lift that the
spring force can be reduced by a factor of about 4 compared to what it
would otherwise be (i.e. without the pinion gear) and still hold the
pivoting member up.

[0024]When the pivoting member 10 is rotated downward (e.g. under manual
force applied by a user) from the up position 14 toward the downward
position (16 in FIG. 1), the drive gear 24 rotates clockwise with the
pivoting member. At the same time, the pinion gear 28 is caused to rotate
counter clockwise by the drive gear, and rotates faster than the drive
gear because of its smaller pitch radius. The beginning of this motion is
shown in FIG. 2. During this motion several things happen simultaneously.
First the spring 38 begins to move toward the center of the two gears. As
this happens, the spring stretches, since the spring attachment tabs on
the drive gear and pinion gear rotate oppositely away from each other.
This aspect of the operation of the mechanism causes the spring to apply
a greater tensile force to the gears.

[0025]At the same time, however, the rotation of the gears reduces the
length of the moment arms of each of the gears, thus tending to reduce
the torque that the spring 38 imposes upon the gear train. Additionally,
since the pinion gear rotates faster than the drive gear (due to its
smaller pitch radius), its moment arm diminishes faster as the mechanism
approaches the center position. Because of this motion, the force to
rotate the pivoting member downward will decrease and the spring will
rotate over center quickly and cleanly. It will be apparent that when the
device reaches the center position--that is, when the spring is directly
over the pivot points of the drive gear and pinion gear--the moment arms
of the drive gear and pinion gear will have diminished to zero, and the
torque provided by the spring on the gear train will thus also become
zero. Thus, at the instant that the spring is at the center position, the
spring will impose no rotational torque on the pivoting member.

[0026]Referring to FIG. 3, once rotated over center, the spring 38
provides a force which begins to act in the opposite direction and biases
the pivoting member 10 toward the downward position in the same way that
it biased the pivoting member up when the spring was on the other side of
center. That is, the drive gear 24 acts as a lever so that the spring
exerts a downward force on the pivoting member when the spring is on the
right side of center. At the same time, the spring exerts a
counter-clockwise torque on the pinion gear, which provides an additional
rotational torque on the drive gear, through the gear engagement, which
also drives the pivoting member down until the pivoting member eventually
reaches the fully downwardly rotated position shown in FIG. 4. In this
position both the weight of the pivoting member and the torque of the
spring will help to hold the pivoting member down.

[0027]To rotate the pivoting member 10 back to the raised position, the
opposite steps are taken. That is, the user pushes upwardly on the
pivoting member, against the force of the spring, until the mechanism
again rotates over center, after which the force of the spring will
assist in rotating the pivoting member to the fully raised position, as
shown in FIG. 1. In this position the force of the spring can hold the
pivoting member in that position until it is desired to lower it again.

[0028]The result of this design is a holding-force on the pivoting member
that is an additive combination of (i) the spring's over-center action on
the drive gear's spring mounting location, (ii) the driven pinion's
resultant driving torque acting through the gear teeth, and (iii) the
pinion acting on the spring to increase the spring's angle, therefore
increasing the moment arm on the gear. This design provides a significant
amount force by combining the over-center spring action with a gear and
pinion that increases the degree of rotation of the spring into its
functional position and adds additional lift to the assembly through the
gear train which allows for a much smaller spring to be used. The result
is a small mechanical package with relatively light forces able to
provide sufficient output torque where needed.

[0029]Static free body diagrams of a pivoting member 500, drive gear 600
and pinion gear 700 as used in an embodiment of a gear-enhanced
over-center spring mechanism in accordance with the present disclosure
are shown in FIGS. 5-7. Viewing FIG. 5, the pivoting member 500 is a
lever arm of length Rcd. A downward force Fc (which can include the
weight of the pivoting member) tends to rotate the pivoting member about
the pivot point 502 in the clockwise direction, requiring an oppositely
directed moment Mcd to maintain static equilibrium. That is, for static
equilibrium,

Mc=(Rcd)(Fd) (1)

[0030]Referring to FIG. 6, the drive gear 600 rotates about a pivot point
602 and has the spring attached at spring attachment tab 604. The drive
gear has a gear pitch radius Rg and a spring lever arm length of Rgs. The
pivoting member moment Mcd rotates about the pivot point 602, and is
opposed by a moment produced by the gear force Fp provided by the pinion
gear (700 in FIG. 7) and by a moment provided by the spring force Fs. For
static equilibrium,

Mc=(Rgs)(Fs)(sin θg)+(Rg)(Fg) (2)

In this equation, the angle θg represents the angular deviation
between the spring force vector Fs and the radial axis Rgs of the drive
gear. It will be apparent that this angle will go from some maximum value
on one side of center to zero when the spring reaches the center
location, and will go from zero to a maximum value on the other side of
center, the sum of the maximum angles on each side of center representing
the total angular range of motion of the pivoting member. In one
embodiment, a maximum angular range of travel of about 40° has
been used.

[0031]Referring to FIG. 7, the pinion gear 700 rotates about pivot point
702, and has the spring attached at spring attachment tab 704. The pinion
gear has a gear pitch radius of Rp and a spring lever arm of length Rps.
The spring force Fs produces a rotational moment of the pinion gear about
its center, which is countered by the moment produced by the gear force
Fp. For static equilibrium,

(Rp)(Fp)=(Fs)(sin θp)(Rps) (3)

As with the drive gear, in this equation, the angle θp represents
the angular deviation between the spring force vector Fs and the radial
axis Rps of the pinion gear.

[0032]The three equations presented above can be combined to solve for Fs,
giving the following relationship:

For design purposes, the values of Fcd and Rcd are given. Because this
equation includes many other design variables, a desired angular range
for θgs has been selected, with other design values then selected
in order to minimize Fs.

[0033]As noted above, the gear radius Rg of the drive gear can be
significantly larger than the radius Rp of the pinion gear. At the same
time, the length of the pinion spring lever arm Rps can be significantly
larger than the length of the drive gear spring lever arm Rgs. This
approach helps to minimize Fs while providing a smooth action that moves
over-center quickly. It is to be appreciated that while the embodiment
shown herein provides a pinion lever arm Rps that is larger than the
drive gear lever arm Rgs, different relative lever arm lengths can be
used. For example, in other embodiments, the two lever arms can be equal,
or the drive gear lever arm can be larger. A variety of combinations of
drive gear and pinion gear radii, in combination with different lever arm
lengths can be used in a gear-enhanced over-center spring mechanism in
accordance with this disclosure.

[0034]A flip-down CD tray having an embodiment of the gear-enhanced
over-center spring mechanism has been built and tested with the following
dimensions and values:

Using these dimension and force values, the nominal spring force Fs to
adequately bias the pivoting member up is about 14.7 N. In contrast, the
same force would have to be about 61.5 N without the gear train. In
addition to the relatively low spring force, the small radius and long
lever arm of the pinion gear causes the spring to move over center
quickly. Consequently, in this embodiment the CD tray snaps into the up
position from about 10 degrees away from the center of travel, and snaps
into the down position from about 10 degrees away from the center of
travel.

[0035]An over-center spring mechanism configured in accordance with the
present disclosure can be used in many applications. As noted above, one
application is in a moveable CD tray for a printer device. Such a printer
device is shown in FIGS. 8 and 9. This printer device 800 is an ink jet
printer, and is capable of printing images and indicia on compact disks
(CDs), digital video disks (DVDs) and the like.

[0036]The printer 800 includes a combined paper tray 802 and output tray
804 that are used to supply paper or other media for printing, and to
receive the completed prints during normal operation. This printer also
includes a CD tray 806 that is configured to be moved to a raised
position, as shown in FIG. 8, whenever the printer is being used to print
upon media from the paper tray. However, when it is desired to print upon
a CD or the like, the CD tray can be rotated downward to the position
shown in FIG. 9. In this lowered position, a CD carrier 808 can be
inserted into the CD tray. In this position, a user can place a CD or DVD
in the circular CD slot 810 of the CD tray, and push against the handle
812 of the CD carrier to slide the tray into the printer to place the
disk in the proper position for printing. When printing is completed, the
user pulls the CD carrier out and removes the printed CD.

[0037]In order to allow the CD tray 802 to be conveniently available for
use, yet be out of the way for normal printing, an over-center spring
mechanism can be used to bias the CD tray between the upwardly rotated
position of FIG. 8, and the downwardly rotated position of FIG. 9. In an
application such as a flip down CD tray for a printer device, the drive
gear and pinion gear can be made of low cost polymer materials. While
other mechanisms are also sometimes used for this type of application,
the gear-enhanced over-center spring mechanism disclosed herein has a
good ability to absorb tolerance variations and material creep over time.
This can allow the use of low-cost materials for the CD tray and
surrounding functional parts.

[0038]The over-center gear and pinion apparatus disclosed herein compounds
the force and over-center gain of an over-center spring mechanism, and
also reduces the non-concentric static loading on the system due to
reduced spring tension. The system provides a gain due to the
relationship of the spring, the drive gear and the driven pinion. In the
embodiment shown herein, there are two factors working together to
increase the system gain in this respect. First, the exaggerated
off-center mounting of the spring increases the spring over-center travel
distance by attaching the ends of the spring to off-centered mounts on
both the drive gear and the pinion. This increases the lateral distance
the spring-ends travel for a given angular displacement of the gear.
Second, because the pitch diameter of the driven pinion is substantially
smaller than the pitch diameter of the drive gear, this causes the driven
end of the spring to pass through a larger arc than the driving end,
which adds even more lateral displacement.

[0039]This system also produces a torque gain due to the spring acting on
the driven pinion. The drive gear starts the transfer of torque by
interacting with the pinion via the gear teeth. As the spring passes over
the center-line of the axis of rotation of the drive gear and pinion
system, the spring begins to apply a moment to the pinion in the
direction of rotation. This developing rotational torque is transferred
back to the drive gear via the gear teeth. The result is a holding-force
on the CD-Tray that is an additive combination of the spring's
over-center action on the drive gear's spring mounting location, and the
pinion's resultant driving torque acting through the gear teeth. In this
particular design the spring force size was able to be reduced by a
factor of about 4 compared to other designs.

[0040]This over-center spring mechanism provides a significant amount of
force by combining an over-center spring action with a gear and pinion
that increases the degree of rotation of the spring into its functional
position and adds additional lift to the assembly through the gear train,
which allows for a much smaller spring to be used. The result is a small
mechanical package with relatively light forces able to provide
sufficient output torque where needed. It snaps into the up position from
about 10 degrees away and snaps into the down position from about 10
degrees away, and avoids creep, part variations, and other issues.

[0041]It is to be understood that the above-referenced arrangements are
illustrative of the application of the principles disclosed herein. It
will be apparent to those of ordinary skill in the art that numerous
modifications can be made without departing from the principles and
concepts of this disclosure, as set forth in the claims.